The present invention relates to a dimming controller for backlight system, and more particularly relates to a twin dimming controller which can adjust the peaked lamp current and operation period simultaneously.
Recently, to follow the fast progress of technologies and increasing application aspects of portable computer, the demand of portable computer is increased substantially, and thus the required standards of size, weight and battery lifetime of portable computer are relatively stricter. In a liquid crystal display (LCD) panel, a backlight controller is utilized to control the dimming operation of cold cathode fluorescent lamp (CCFL), and normally, a conventional analog dimming control (ADC) method is utilized to adjust lamp current for adjusting brightness. However, the backlight cannot be adjusted to be too dim, or uneven brightness will be resulted in. Thus, backlight power source cannot be set too low, which means that the working time of portable computer cannot last very long outdoors. In order to extend the working time of portable computer, many power-saving modes about backlight power source have been researching currently.
One of the most frequently improvement methods adapted is a pulse dimming control (PDC) method, or called burst mode control method. Basically, the PDC method utilizes a pulse modulation method to adjust the working period of lamp current for controlling the panel brightness. Please referring to FIG. 1, FIG. 1 is a diagram showing the working period of lamp current generated by PDC method. With 100% working period of pulse width, the outputted lamp current is at the maximum, and with the decreased working period, the working period of lamp current also is decreased, leading to less the panel brightness. Theoretically, with a shorter working period of pulse width, the panel brightness is lower, and thus the input power required by backlight controller is smaller. However, it is a disadvantage of PDC method that a flicker phenomenon of image is likely to be induced by the insufficient pulse modulation width caused by an overly short working period of pulse. The bottom line of safety for working period of pulse width is about 20% to 30%.
Please referring to FIG. 2, FIG. 2 is a diagram showing the circuit of a conventional analog dimming controller. As shown in FIG. 2, the circuit is mainly divided into a buck stage 100, an error-feedback amplifier and frequency compensation loop 200, and a resonant push-pull stage circuit 300. The resonant push-pull stage circuit 300 is constructed with a transformer T1, a resonant capacitor CR, and two interactive switch transistors Q2 and Q3. The resonant frequency is determined by the primary inductance LP of transformer T1, turn ratio N, resonant capacitor CR and capacitor N2CB returned from the secondary side of transformer T1,       f    r    ≈      1          2      ⁢      π      ⁢                                    L            p                    ⁡                      (                                          C                R                            +                                                N                  2                                ⁢                                  C                  B                                                      )                              
wherein CB is ballast capacitor of backlight controller.
As shown in FIG. 2, the analog dimming control is utilized in conventional CCFL, and the lamp current thereof is a light-adjusting method of continuous output. Generally, a DC average is obtained by first connecting a variable resistor RV1 and a fixed resistor RS for forming an induced current, and then via a rectifiable diode D2. Then the DC average is inputted to the error-feedback amplifier and frequency compensation loop 200 for adjusting the lamp current. Thus, the lamp current is       I    LAMP    =                    [                              V                          REF              ⁢                              xe2x80x83                            ⁢              1                                +                                    V              D                        2                          ]            ⁢      π                      2            ⁢              (                              R                          V              ⁢                              xe2x80x83                            ⁢              1                                +                      R            S                          )            
The aforementioned analog dimming control method is frequently limited by the minimum lamp current so as to prevent uneven brightness from occurring. Therefore, the lamp current cannot be adjusted too small, whereby the dimming range is limited.
Hence, a backlight control circuit with pulse dimming control shown in FIG. 3 is implemented widely. Please referring to FIG. 3, FIG. 3 is a diagram showing a backlight control circuit with pulse dimming control. By utilizing an additional pulse width modulation (PWM) modulator 400, the pulse generated by PWM modulator 400 enters the input terminal or the output terminal of error-feedback amplifier and frequency compensation loop 200, so that the CCFL current has a regular switch ON/OFF interval in accordance with the change of working period under a fixed low frequency, thereby efficiently decreasing the valid value of lamp current to achieve the objective of changing the brightness of lamp. At this time, the lamp current is outputted under a PWM mode, but the peak value of lamp current is not changed, so that with the shorter working period, the smaller lamp current is smaller accordingly, and the overall consumptive power of backlight controller can be maintained at a minimum status for attaining the goal of power saving.
Please referring to FIG. 4, FIG. 4 is a diagram showing the working period of lamp current that is less than 20%. If the lamp current is smaller than 1.2 mA, the working period of lamp current is smaller than 20% or more, so that the switch-ON time of lamp current is less than the soft-starting time of buck stage 100. Thus, the lamp current becomes unstable, and the entire LCD image is likely to appear a flicker phenomenon. As shown in FIG. 4, the working period of lamp current that is less than 20% indicates the unstable status. Therefore, the smallest working period has to be set at 20% or over 30% to avoid this flicker phenomenon, so that the dimming range is limited and can only reaches 1:10, and the minimum power consumption of lamp can be only down to 0.6 W, and if the panel brightness of LCD is lower than 10 Nits, the flicker phenomenon occurs.
In the view of the background of the invention described above, there are some disadvantages, such as that the dimming range of backlight controller with PDC is limited to 1:10, and the minimum power consumption of lamp is 0.6 W. Therefore, the present invention proposes a twin dimming controller for backlight system to resolve the aforementioned problems.
It is the principal object of the present invention to provide a twin dimming controller for backlight system. In the present invention, the output of PWM modulator is used as the referenced voltage of error-feedback amplifier and frequency compensation loop, and is connected to the output terminal of error-feedback amplifier and frequency compensation loop at the same time. Thus, the present invention can adjust the peaked lamp current and the working period of pulse width for adjusting the output power of lamp.
It is another object of the present invention to provide a twin dimming controller for backlight system. By utilizing the present invention, the dimming range is broader with the same working period of lamp current, and the minimum power consumption of lamp is decreased substantially.
In accordance with the aforementioned purpose of the present invention, the present invention provides a twin dimming controller for backlight system, comprising: a buck stage, a resonant push-pull stage circuit connecting with the buck stage power for generating a lamp current having resonant frequency, a ballast capacitor connected in series with CCFL between the output terminal of resonant push-pull stage circuit and the input terminal of error-feedback amplifier and frequency compensation loop, a resistor connected in parallel with a first diode between the output terminal of resonant push-pull stage circuit and a ground voltage, an output terminal of PWM modulator connected to an output terminal of error-feedback amplifier and frequency compensation loop, a n-type terminal of second diode connected to the output terminal of PWM modulator and a p-type terminal of second diode connected to an referenced voltage of error-feedback amplifier and frequency compensation loop.
In accordance with the aforementioned purpose of the present invention, the present invention provides a twin dimming controller for backlight system, the characteristic of the present invention is the PWM signal outputted from PWM modulator of backlight controller having PDC sent to the n-type terminal of diode, and the p-type terminal of diode connected to the reference terminal of error-feedback amplifier and frequency compensation loop.